If constructing your own scripts, you can find your monitor output names (LVDS1 and VGA1 for me) by running xrandr without any arguments. man xrandr is helpful for figuring out the options you can use to get the layout like you want it.

This workaround works great for me, eliminating the need to solve the initial problem.

Speedy work! I don't have any experience with Dells, but I hope you find a solution.
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Chris HarcourtNov 9 '09 at 23:15

That's a good idea. I tried scanning for the keyboard event with xev, but xev does not receive any events when that key combination is pressed. Unfortunately, that led nowhere.
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nilbusNov 10 '09 at 3:28

So, I know this doesn't answer your question - yet. I was trying to find what code cycles through the 5 "modes" you mention in your question, and this is what I've found so far.

It looks like you've found a bit of a can of karmic worms.

It appears that the Fn-F8 key is getting ignored within dell-wmi.c - I see an error to /var/log/messages each time I press it: dell-wmi: Unknown key 42 pressed - so the code to switch monitor configurations isn't being called from there.

With nothing plugged into the external VGA port, here's what I'm seeing in /var/log/messages when I hit Fn-F8: